Chapter 4: Results
4.1.1 Range of motion (ROM)
In extension, the ROM increased 64 % in the defect model at the surgical segment (Figure 4.1). After implantation, the ROM effectively decreased 70 % in the Coflex model, 76
% in the Coflex-F model, and 90 % in the pedicle screw fixation model as compared with the intact model. In addition, the ROM increased 24 % in the Coflex and Coflex-F models at the adjacent L1-L3 segments and increased 20 % at the adjacent L4-L5 segment. The ROM increased 19 % in the pedicle screw fixation model at the adjacent L1-L3 segments and increased 25 % at the adjacent L4-L5 segment.
In flexion, the ROM increased 13 % in the defect model and 8 % in the Coflex model at the surgical segment (Figure 4.2). In contrast to the above two models, the ROM decreased 52
% in the Coflex-F and 68 % in the pedicle screw fixation models at the surgical segment. On the other hand, the ROMs of the defect model and the Coflex model were similar to that of the intact model at both adjacent L1-L3 (deviation within 4 %) and L4-L5 segments (deviation within 4 %). However, the ROM increased 17 % to 18 % in the Coflex-F model and 23 % to 24 % in the pedicle screw fixation model at both adjacent L1-L3 and L4-L5 segments.
In lateral bending, the ROM decreased 8 % in the Coflex model, decreased 20 % in the Coflex-F model, and decreased 51 % in the pedicle screw fixation model at the surgical segment as compared with the intact model (Figure 4.3). The ROMs of the Coflex and Coflex-F models were similar to that of the intact model at both adjacent L1-L3 (1 % to 2 %)
and L w fixation m In axial rot ex-F model ment as com
ex-F model model at bot tation, the R
l, and decr mpared with ls were sim nd L4-L5 s the ROM i -L3 segmen
gure 4.1: Ran
to 2 %). Ho th adjacent L ROM decrea reased 40 % the intact m milar to that egments (d increased 2 nt, and incre
nge of moti
owever, the L1-L3 and ased 4.3 %
% in the pe model (Figu
t of the inta deviation w
0 % at the eased 14 %
ion normali
ROM incre L4-L5 segm
in the Cofl edicle screw
re 4.4). The act model a within 2 %) adjacent L at the adjac
zed to intac
eased 16 % ments.
lex model, d w fixation e ROMs of t at both adja
. However, the defect, C acent L1-L3
Fi
Figure
gure 4.2: R
e 4.3: Range
Range of mo
e of motion
58
otion normal
n normalized
lized to inta
d to intact m
act model in
model in late
n flexion.
eral bendingg.
4. In flexion, ical segmen
el, the max cle screw f
re 4.4: Rang
al von-Mise on, the maxi
model, and th the intact models was in 2 %). Th
% at the adj maximal disc
-L2 segmen nt L4-L5 seg
the maxim nt as compa ximal disc a fixation mo
ge of motion
es stress at t imal disc an d 79 % in th t model (Fig similar to t he maximal jacent L2-L c annuls stre nt, increased
gment.
mal disc ann ared with th annulus str
dels. On th
n normalize
the disc ann nnulus stres
he pedicle s gure 4.5). T he intact m ress decreas he other ha
ed to intact m us stress of t and decre edicle screw he adjacent
s increased model (Figur sed 15 % in and, the ma
model in ax
d 75 % in th ion model a l disc annul el at the adj f the Coflex
ased 4 % a w fixation m L2-L3 segm
5 % in the re 4.6). In
n the Cofle aximal disc
xial rotation
he Coflex m at the surgic lus stress of djacent L1-L x and Coflex at the adjac model increa ment, and in
e Coflex m contrast to ex-F and 2
annulus st n.
model, 81 % cal segment f the Coflex L2 segment x-F models cent L4-L5 ased 7 % at ncreased 18
model at the the Coflex 7 % in the tress of the
%
60
Coflex model was similar to that of the intact model at both adjacent L1-L3 and L4-L5 segments (deviation within 4 %). However, the Coflex-F and pedicle screw fixation models increased maximal disc annulus stress by 18 % to 22 % at both the adjacent L1-L3 and L4-L5 segments.
In lateral bending, the maximal disc annulus stress decreased 18 % in the Coflex model, 25 % in the Coflex-F model, and 41 % in the pedicle screw fixation model at the surgical segment as compared with the intact model (Figure 4.7). The maximal disc annulus stress of the Coflex and Coflex-F models decreased 6 % to 8 % at both adjacent L1-L3 and L4-L5 segments. However, the maximal disc annulus stress of the pedicle screw fixation model increased 15 % to 21 % at both adjacent L1-L3 and L4-L5 segments.
In axial rotation, the maximal disc annulus stress decreased 15 % to 16 % in all implanted models at the surgical segment as compared with the intact model (Figure 4.8). The maximal disc annulus stress increased 11 % in the Coflex and Coflex-F models, and 7 % in the pedicle screw fixation model at the adjacent L1-L2 surgical segment. The maximal disc annulus stress of all implanted models increased 15 % at the adjacent L2-L3 segment. The maximal disc annulus stress of the Coflex model and Coflex-F model were similar to that of the intact model at the adjacent L4-L5 segment (deviation within 2 %). The maximal disc annulus stress of the pedicle screw fixation model increased 19 % at the adjacent L4-L5 segment.
Figu
Fig
ure 4.5: Disc
gure 4.6: Dis
c annulus st
sc annulus s
tress normal
stress norma
lized to inta
alized to int
act model in
tact model i
n extension.
in flexion.
Figure
Figure
4.7: Disc an
e 4.8: Disc a
nnulus stres
annulus stre
62
ss normalize
ess normaliz
ed to intact
zed to intact
model in la
t model in a
ateral bendin
axial rotatio ng.
n.
4.1.3 Von-Mises stress distribution at disc annulus
The stress concentration and distribution pattern of the disc annulus at the surgical segment (L3-L4) changed obviously in these models. In extension, the stress of the defect model was concentrated at the posterior inferior regions of the annulus (Figure 4.9 middle).
However, after implantation, the stress concentration of the disc annulus at the posterior disc diminished obviously. Furthermore, in flexion, the stress concentrated at the anterior of the annulus regions, close to the superior and inferior sides of the endplate, in both defect and Coflex models as compared with the intact model (Figure 4.10 middle). The Coflex-F and pedicle screw fixation models have the most even disc annulus stress distribution in flexion, even when compared with the intact model. In lateral bending and in axial rotation, the stress was concentrated at the right part of the annulus regions, close to the superior and inferior sides of the endplate in the defect model as compared with the intact model (Figures 4.11 middle, 4.12 middle). After implantation, the stress concentration of the disc annulus at the posterior disc also diminished.
The stress distribution pattern of the disc annulus at adjacent segment (L2-L3, L4-L5) was affected in these models. In extension, stress of disc annulus decrease in defect model;
stress increase in implantation model, compared with the intact model (Figure 4.9 top and bottom). In flexion, stress distribution of disc annulus in both defect and Coflex models was close to the intact model (Figure 4.10 top and bottom). However, after implantation, stress concentrated at the anterior of the annulus regions, close to the superior and inferior sides of the endplate, in both Coflex-F and pedicle screw fixation models as compared with the intact model. In lateral bending and in axial rotation, stress distribution of disc annulus in both defect, Coflex, Coflex-F models was close to the intact model (Figures 4.11 top and bottom, 4.12 top and bottom). After pedicle screw fixation, the stress was concentrated at the right part of the annulus regions.
Fi mo
igure 4.9: vo odels: (Top)
on-Mises st L2-L3 adja
tress distribu acent segme
64
ution of dis ent; (Middle
adjacent seg
c annulus in e) L3-L4 su
gment.
n extension rgical segm
for various ment; (Botto
s surgical m) L4-L5
F mo
igure 4.10:
odels: (Top)
von-Mises L2-L3 adja
stress distri acent segme
ibution of d ent; (Middle
adjacent seg
disc annulus e) L3-L4 su
gment.
s in flexion f rgical segm
for various ment; (Botto
surgical m) L4-L5
Figu surg
ure 4.11: vo gical models
on-Mises str s: (Top)
L2-ress distribu -L3 adjacen
L4-66
ution of disc nt segment;
-L5 adjacen
c annulus in (Middle) L t segment.
n right latera 3-L4 surgic
al bending f cal segment
for various t; (Bottom)
Fig surg
gure 4.12: v gical models
von-Mises st s: (Top)
L2-tress distrib -L3 adjacen
L4-bution of dis nt segment;
-L5 adjacen
sc annulus i (Middle) L t segment.
n right axia 3-L4 surgic
al rotation fo cal segment
or various t; (Bottom)
68
The pedicle screw fixation procedure frequently associated with postoperative long-term complication of adjacent segment disease, resulting in the cause of another surgery for extended pedicle screw fixation at the adjacent segments. The higher incidence of adjacent segment disease was reported when patient was treated with rigid instrumentation.
According to the above ROM and von-Mises stress distribution of disc results, the Coflex device can provide stability in extension, lateral bending, and axial rotation at the surgical segment and retain flexible in flexion. The Coflex device restraint extension motion, and provide more space of foramen and spinal canal. Besides, it had no influence than pedicle screw fixation at adjacent segments except during extension. Therefore, the use of Coflex device may decrease rate of adjacent segment disease. As a result, it may replace pedicle screw fixation to improve stenosis in surgical and complication in adjacent segments.
4.2 C
Coflex-F in Biomechan ex-F model pared with el as percen
2.1 Range o For the Co eased by 75 ding, left la
parison with those of th ntage values
of motion oflex-F com
5 %, 81 %, ateral bendi h the intact M at the sur ysiological m
edicle screw 3 %, 88 % or the pedi
sed by 83 tively.
ery
iors of the LIF and AL he intact m s under each
mbined with , 35 %, 47 ing, right a t model (Fi rgical segme motions, res w fixation
%, 68 %, 7 icle screw
%, 90 %, 7
lumbar spi LIF combin model. Data h loading co
TLIF, rang
ne with the ned with pe were norm ondition.
ge of motion and 36 % i n flexion, e ft axial rot
.18). For th w fixation m
h respect to
gical segme extension, r tation, resp o the intact
ent (L3-L4)
Fig
Figu
Figure 4.
Figure 4
gure 4.13: R
ure 4.14: Ra
15: Range o
.16: Range
Range of mo
ange of mot
of motion no
of motion n
70
otion norma
tion normali
ormalized t
normalized
alized to int
ized to intac
to intact mo
to intact mo
act model in
ct model in
del in right
odel in left l
n flexion.
extension.
lateral bend
lateral bend
ding.
ding.
4.
ises stress d ncentration
of motion n
e of motion
IFs were co s show no
normalized
normalized
on the cage ibution pat rface of the he stresses w
Coflex-F sh lly when c oncentrated
directional
to intact mo
d to intact m
e-bone inter ttern of str e L4 vertebr
were concen hows more combined w
and correla l effect (Fig
odel in right
model in left
face ress chang ra at the sur ntrated at th
significant with TLIF.
ated with ax gure 4.20).
ht axial rotat
t axial rotati
ged obvious rgical segme he same sid
stresses co In axial ro xial rotation The Coflex
tion.
ion.
sly on the ent for four de as lateral oncentration
resul on, the stre rior side of f vertebra fo
igure 4.21).
ation on the
The von-Mi of the L4 v
t stresses c cage-bone or all impla . In extensio
cage-bone i
ises stress d vertebra und der left later
on then pe f the L4 ver especially f the implan
on the cage ral bending
dicle screw rtebra were
for the Co nt models st
-bone interf and right la
w fixation all concent oflex-F com tress shows
faces of the ateral bendin
models. In trated at the mbined with s significant
e superior ng.
n e h t
Fig
Fig
gure 4.20: T surfac
gure 4.21: T
The von-Mi ce of the L4
The von-Mi surface
ises stress d 4 vertebra un
ises stress d of the L4 v
distribution nder left axi
distribution vertebra und
on the cage ial rotation
on the cage der flexion a
-bone interf and right ax
-bone interf and extensio
faces of the xial rotation
faces of the on.
e superior n.
e superior
4. the pedicle
er stresses t maximum v ing cases. F w, when com
gure 4.22: T fixation
ises stress d 2 shows the
screw for v than the ped von-Mises e contour p arious loadi dicle screw stress value hese cases, h either TLI
ses stress dis on, extensio
74
for the Cofl plots of von ing cases. F when comb es in the C the Coflex IF or ALIF.
stribution in on, right lat
flex-F and th n-Mises stre For all of the bined with T Coflex-F an x-F devise h
n the Coflex teral bendin
he pedicle s ess values i ese cases, th TLIF or AL nd the pedi has higher s
x-F device a g, and right
crew in the Cofle he Coflex-F LIF. Figure
icle screw stress than
and the ped t axial rotati
ex-F devise F devise has 4.23 shows for various the pedicle
icle screw ion.
Figu
Coflex-F in Biomechan el, the TLIF ion model, pared with el as percen
3.1 Range o For the TL eased by 75 ding, left la
parison with F combined and the T those of th ntage values
of motion LIF combine
5 %, 81 %, ateral bendi h the intact le screw fix
m von-Mise he intact m s under each
ed with Cof , 35 %, 47 ing, right a t model (Fig xation and
es stress of t ht lateral ben
sion surgery lumbar spi teral pedicle ined with b model. Data h loading co
flex-F, rang nding, and r
y
ine with th e screw fixa bilateral ped
were norm ondition.
ge of motion and 36 % i on, and lef
~ Figure 4.2 ar facet scre
F device and right axial ro
e TLIF com ation and tra
dicle screw malized with
n at the surg n flexion, e ft axial rot 29). For the
ew fixation
nd pedicle sc otation.
mbined wit anslaminar w fixation m
h respect to
gical segme extension, r tation, resp e TLIF com n, ROM at t
crew under
th Coflex-F facet screw model were o the intact
ent (L3-L4)
segm motio the s phys
ment decrea ons, respec surgical seg
iological m
Fig
Figu
sed by 84 tively. For gment decr motions, resp
gure 4.24: R
ure 4.25: Ra
%, 90 %, 7 the TLIF c reased by 8 pectively.
Range of mo
ange of mot
76
73 %, 67 % combined w
84 %, 89 %
otion norma
tion normali
%, 66 %, an with bilatera
%, 68 %,
alized to int
ized to intac
nd 66 % in l pedicle sc 72 %, 64
act model in
ct model in
n the six ph crew fixatio
%, and 64
n flexion.
extension.
hysiological on, ROM at 4 % in six l t x
Figure 4.2
Figure 4
Figure 4
26: Range o
.27: Range
.28: Range
of motion no
of motion n
of motion n
ormalized t
normalized
normalized
to intact mo
to intact mo
to intact mo
del in right
odel in left l
odel in right
lateral bend
lateral bend
ht axial rotat ding.
ding.
tion.
Figure 44.29: Rangee of motion
78
normalizedd to intact mmodel in left t axial rotatiion.
Chapter 5 Discussion
5.1 Coflex and Coflex-F in non-fusion surgery
The subject study found that (1) the Coflex device can provide stability of the surgical segment in most motions, except in flexion; (2) the rivets of the Coflex-F link bone and implant and can provide stability in all motions, especially in flexion; (3) in flexion, the disc stress distribution of the surgical segment is improved by the use of rivets; (4) in flexion, the Coflex-F influenced the adjacent segments; and (5) in extension, all implants influenced the adjacent segments.
In the subject study, the Coflex device in the defect model was found to provide stability in most motions, except in flexion. The instability of the Coflex device in flexion causes stress concentration at the anterior regions of the disc annulus (close to the superior and inferior sides of the endplate). Wilke et al. [59] suggested that the key for the Coflex device to provide stability in flexion is based on whether the teeth on the wings of the Coflex can provide sufficient anchorage to the spinous process. Two factors can improve this stabilization effect.
First, the surgeon must tighten the teeth on the wings against both edges of the spinous processes. Second, the bone density of the spinous processes should be strong enough to provide sufficient anchorage. However, both conditions are not always guaranteed.
For numerical analysis, the coefficient of friction in the interface between the implant and spinous processes was difficult to obtain. It is hypothesized that the teeth on the wings of the Coflex device will prevent implant slip motion in the spinous processes, and therefore a higher coefficient of friction (0.8) was used in the contact interface. In addition, this study also tested different coefficients of friction (0.4, 0.8, 1.2, and 1.6) to seek its influence on the effect of teeth on the wings of the Coflex device. The results show that the influence of the coefficient of friction is negligible.
The Coflex device was implanted between the interspinous processes located at the
80
posterior structure of the spine to resist instability in extension. By comparison with Tsai’s results in cadaveric experiments [57], our data show discrepancies in lateral bending and axial rotation. It is inferred that these were caused by individual differences among cadaveric specimens and differing experimental conditions. In the subject study, a partial L3-L4 interspinous process was removed to provide sufficient space for the implant, and the spinous process interface was modeled as a perfect contact and was able to transmit both tensile and compression forces. This assumption is different from the results of cadaveric experiments.
Kettler et al. [55] reported implantation of the Coflex-F can provide stability for all motions in lumbar spine stability. In the subject study, we also showed that the rivet connecting the metal wings and bony spinous process provides more security than the conventional Coflex device. Therefore, the rivet can improve the load transmission on the posterior spinal structure to decrease the stress concentration on the disc annulus at the surgical segment in all motions.
There are limited reports about implanting the Coflex device in the long lumbar segment model. The potential side effects in the adjacent segments need to be addressed. In 1-year outcome evaluation, Kong et al. [58] reported that the Coflex device reduced the ROM at the surgical segment but did not affect the ROM at the adjacent segments. The subject study, using a long lumbar spine segment model of an implanted Coflex device, showed that the ROMs are increased at both adjacent segments in extension but are unchanged in other motions. Therefore, the Coflex device increased annulus stress at both adjacent segments in extension. However, the Coflex-F constrained the surgical segment in all motions and it increased ROM at adjacent segments, especially in flexion. Therefore, the Coflex-F increased annulus stress at both adjacent segments in flexion and extension. The Coflex-F and pedicle screw fixation have the same effect on the adjacent segments in both flexion and extension. In addition, the remote adjacent L1-L2 segment and adjacent L2-L3 segment demonstrate the same effect in all forms of implantations.
5.2 Coflex-F in fusion surgery
The subject study found that (1) The Coflex-F device combined with ALIF can provide stability similar to a pedicle screw fixation in combination with TLIF or ALIF. (2) larger stress at the cage-bone interface for the Coflex-F combined with TLIF, thus causing the exclusion of the pedicle screw fixation.
The present study used an FE lumbar model of the L1-L5 segments to compare the effects of the Coflex-F device and traditional bilateral pedicle screw fixation at the surgical segment after TLIF and ALIF implantation. According to the ROM results, the Coflex-F device combined with the TLIF model had lower stability than all the other models, especially in both directions of lateral bending and axial rotation. On the other hand, the pedicle screw
The present study used an FE lumbar model of the L1-L5 segments to compare the effects of the Coflex-F device and traditional bilateral pedicle screw fixation at the surgical segment after TLIF and ALIF implantation. According to the ROM results, the Coflex-F device combined with the TLIF model had lower stability than all the other models, especially in both directions of lateral bending and axial rotation. On the other hand, the pedicle screw